Local intercom circuits conventionally have been provided to enable multiple parties to signal each other and to communicate with each other without invoking any incoming line and its related central office line finger equipment. Such local intercom circuits have been provided typically as a functional feature of key telephone systems. Such systems are characterized by multiple incoming telephone lines and multiple user telephone station instruments being connected to the lines so that each instrument has key button access to each of the incoming lines. Such conventional intercom circuits, whether half-duplex or full duplex, have required centralized signalling and communication circuits and special local lines dedicated to the intercom function. These circuits have been expensive to make, install and operate.
More recently, complex digitized voice systems using the telephone line, and AM and FM local area carrier systems via primary power line distribution wiring, havve been employed. Although advances have occured in connection with the integration of FM communications circuits into small semiconductor packages, a need has heretofore remained unsolved for an improved full duplex telephone intercom system which may be configured to use existing local telephone circuit wiring without tying up incoming telephone line pairs from the central office, or requiring the installation of extra dedicated intercom pairs.
The hitherto unsolved need includes the provision of a fully self-contained, low cost intercom circuit within a telephone instrument which enables full duplex communication (i.e. simultaneous transmission and reception) without requiring any additional pairs of wire in the local cable. In situations where two or more pairs of wires are available locally for the telephone circuits, as with two or more line conventional key telephone systems, a hitherto unsolved need for a still more simplified and cost reduced intercom circuit arrangement for telephone systems without special intercom cabling has continued to exist.